Generally, in a liquid crystal display device for producing a color display, one pixel is divided into three sub pixels: a red sub pixel provided with a color filter that transmits red light, a green sub pixel provided with a color filter that transmits green light, and a blue sub pixel provided with a color filter that transmits blue light. The color display can be produced by the color filters provided in these three sub pixels. However, in recent years, for the purpose of expanding a color space (a color gamut or a color reproduction range), there has also been developed a liquid crystal display device in which one pixel includes a white sub pixel that transmits white light, and the above three sub pixels (i.e., a liquid crystal display device in which one pixel includes the white sub pixel, the red sub pixel, the green sub pixel, and the blue sub pixel).
Further, since the liquid crystal display device employing a color filter system as described above has the problem of having low light use efficiency, a liquid crystal display device employing a field-sequential color system in which a color display is produced without using color filters has also become widespread. In a typical liquid crystal display device adopting the field-sequential color system, one frame period being a display period for one screen is temporally divided into three fields. While the field is also referred to as a subframe, in the following description, the term “field” is used uniformly.
In the liquid crystal display device employing the field-sequential color system, typically, one frame period is temporally divided into a field (red field) for displaying a red screen based on a red component of an input video signal, a field (green field) for displaying a green screen based on a green component of an input video signal, and a field (blue field) for displaying a blue screen based on a blue component of an input video signal. By displaying the primary colors one by one as above, a color image is displayed on a liquid crystal panel. Displaying a color image in such a manner eliminates the need for color filters in the liquid crystal display device employing the field-sequential color system. Accordingly, the liquid crystal display device employing the field-sequential color system has high light use efficiency as compared with that of the liquid crystal display device employing the color filter system. Hence, the liquid crystal display device employing the field-sequential color system is suitable for increasing luminance and reducing power consumption.
In the liquid crystal display device employing the field-sequential color system described above, a field (white field) for displaying a white screen is provided in addition to the above three fields in order mainly to reduce color breakup.
As described above, in the liquid crystal display device employing the color filter system, the white sub pixel is provided so as to expand the color space, while in the liquid crystal display device employing the field-sequential color system, the white field is provided so as mainly to reduce color breakup. Meanwhile, a signal value for white is decided based on a signal value for red, a signal value for green, and a signal value for blue. At that time, an expansion process for increasing signal values for red, green and blue is performed so as to expand the color space. Generally, the expansion process is performed by multiplying an original signal value for each of red, green, and blue by a constant coefficient (hereinafter referred to as an “expansion coefficient”).
Japanese Patent Application Laid-Open No. 2010-33009, for example, discloses an invention of an image display device in which one pixel is configured by four sub pixels (a red sub pixel, a green sub pixel, a blue sub pixel, and a white sub pixel) to expand a color space. In the image display device disclosed in Japanese Patent Application Laid-Open No. 2010-33009, “the maximum value of lightness (the maximum lightness)” with saturation taken as a variable is previously stored into a signal processing unit, and an expansion coefficient is decided based on saturation obtained from an input video signal and the maximum lightness stored in the signal processing unit. The expansion coefficient is then used to perform the expansion process on the input video signal. In such a manner, the color space (HSV color space) is expanded from one as shown in FIG. 20 to one as shown in FIG. 21.